The invention relates generally to the fabrication of structures from fiber-reinforced composite materials and, more particularly, to methods and apparatuses that utilize a fluid bearing to consolidate or lay such materials.
In recent years the use of fiber-reinforced resin composites has increased considerably, particularly in aerospace sturctures where high strength-to-weight characteristics are extremely desirable. Various methods have been devised to form laminated structures using composite materials in the form of sheets or tapes of woven or aligned fibers in a resin matrix. In those methods that use sheet goods as the working material, the laminated structures are formed by stacking or laying up a plurality of plies, with the ply direction and number of plies determined on the basis of the intended use of the resultant structure. In other processes, composite tapes are laid side by side by hand or by automated tape-laying machines in first one direction and then in another direction to form multiple layers having cross-oriented fibers.
After a layup is formed in one of these manners, it is placed inside an autoclave or a press and subjected to heat and pressure. In the case of thermosetting materials, the heat causes the resin to cure; whereas, in the case of thermoplastic materials, the heat raises the resin above its melt temperature. The structures produced in this manner are inherently costly because of the manual handling of the material and the length of time required for the autoclave to cycle.
As a result of these difficulties, efforts have more recently focused upon developing cost-effective methods to process composites having thermoplastic or thermosetting resins. Thermoplastic materials are particularly desirable since they need only be heated to their melt temperature (and not cured) and can be repeatedly softened and solidified. Because of these characteristics, it is possible to form sheet stock, or preconsolidated multiple-ply laminates, which are subsequently formed or machined into the final product or part. Several proposals have been made to utilize point contact, such as via belts, pressure rollers, or shoes, to provide the pressure needed for consolidation. To supply the necessary heat, suggestions have been made to heat the material just before entering the pressure area or heat the material while in the pressure area via the use of a heated roller or hot shoe. There are a number of disadvantages with these approaches. First, frictional loads are undesirably applied to the material as it moves relative to the rollers, belts, or shoes. Secondly, it is difficult to precisely control the speed of the material through the pressure area in relation to the melting cycle so that the material properly solidifies under a uniform pressure before leaving the pressure area. Thirdly, with ultimate contact between the machine and material, problems of buildup and release can occur.
A fourth, and important, disadvantage is that these methods are limited to the formation of continous sheets or other parts having a substantially constant thickness. Many parts, particularly those used in aerospace applications, do not have a constant cross-sectional configuration or uniform thickness. For example, a wing skin generally tapers in thickness in a spanwise direction from the root to the tip. When composite materials are used to form such tapering parts, the necessary taper is produced by gradually dropping off, or reducing, the number of plies. The step-like "ply drop-offs" that occur where a ply is discontinued produce an irregular contour and an incremental change in thickness. The foregoing arrangements rely upon a fixed gap in the pressure area and thus cannot accommodate the ply drop-offs or follow the irregular contour.